It is well known in the field of optical communications that coherent optical heterodyne receivers provide, in principle, the best available performance. Such receivers utilise a local optical oscillator, ie a suitable laser located at the receiver, having a frequency nominally equal (or almost equal) to that of the optical signal to be detected. The output field from the local laser is combined with the received signal field, and the combined signal directed to an optical-to-electrical converter, such as a photodiode. The process of so-called “square-law detection” (ie conversion of optical intensity into electrical current or voltage) causes mixing between the local oscillator field and the received signal field, whereby the optical signal is converted to an electrical signal within the radio frequency (RF) domain. The performance of this type of optical receiver is, in theory, limited only by the fundamental quantum noise fluctuations occurring within the local oscillator field. Consequently, continuous improvement in receiver noise performance may be obtained by increasing local oscillator power, up to the limits of the components utilised in the receiver.
Despite these excellent theoretical performance characteristics, coherent optical heterodyne receivers are rarely used in optical communications systems. This is in part due to the development of practical, high-gain, low-noise, optical amplifiers, which may be used to construct pre-amplified optical receivers having a noise figure that is, in principle, within 3 dB of the theoretical optimum noise performance of a coherent optical heterodyne receiver. Furthermore, in the presence of amplified spontaneous emission (ASE) noise generated in optical amplifiers used within optical communications links, receiver noise may no longer represent the ultimate limit to overall performance of the transmission systems. In these circumstances, the benefits available from the use of optical heterodyne receivers are substantially eroded. Most importantly, however, coherent optical heterodyne receivers are generally considered to have a number of practical disadvantages, particularly in terms of cost and complexity, which preclude their widespread deployment in optical communications systems.
Perhaps the main disadvantage of coherent optical heterodyne receivers is the requirement to provide a local optical oscillator having a narrow linewidth and a stable output frequency that remains locked to the frequency of the received optical signal. This generally requires that appropriate frequency control techniques be applied to the local oscillator laser. Furthermore, coherent optical detection is an inherently polarisation-sensitive process, since mixing only occurs at the photodetector between co-polarised components of the received signal and the local oscillator. Although it is possible, in principle and in practice, to align the polarisation states of the local oscillator and the incoming optical signal, additional complications arise in the case of very high bandwidth signals that have been transmitted over long, unregenerated optical links in which significant polarisation mode dispersion (PMD) may occur. In particular, PMD is a time- and frequency-dependent phenomenon, and it may therefore be difficult, or impossible, to align the polarisation state of all components of the received optical signal with that of the local oscillator.
For at least these reasons, most receivers deployed in optical communications systems are incoherent, or direct-detection receivers. The performance of such receivers is generally acceptable, and near optimum, for baseband optical transmission systems based upon intensity modulation (ie IM/DD systems). However, direct-detection receivers may not provide adequate performance in emerging applications employing alternative information coding and modulation formats. For example, a number of co-pending patent applications sharing common inventorship with the present application describe systems utilising orthogonal frequency division multiplexing (OFDM), and related techniques, for transmission of signals over optical channels. In such systems, the use of direct-detection may result in frequency-dependent attenuation, or fading, in the detected electrical spectrum, due to processes such as chromatic dispersion and PMD. In such cases, the use of a coherent optical heterodyne receiver may be beneficial, were it not for the associated cost and complexity.
It would therefore be desirable to provide an alternative apparatus for receiving an optical signal, and an associated method of reception, which is able to achieve at least some of the benefits of coherent optical heterodyne receivers, while avoiding the complexity associated with a local optical oscillator, and which therefore better meets the needs associated with emerging transmitter technologies, utilising coding and modulation methods other than traditional baseband intensity modulation.
The present invention seeks to address this need.